1. Technical Field
A system and methods for extending the frequency bandwidth of harmonic signals are provided.
2. Prior Art
All communication systems, especially wireless communication systems, suffer bandwidth limitations. The quality and intelligibility of speech signals transmitted in such systems must be balanced against the limited bandwidth available to the system. In wireless telephone networks, for example, the bandwidth is typically set according to the minimum bandwidth necessary for successful communication. The lowest frequency important to understanding a vowel is about 200 Hz and the highest frequency vowel formant is about 3,000 Hz. Most consonants however are broadband, usually having energies in frequencies below about 3,400 Hz. Accordingly, most wireless speech communication systems are optimized to pass between 300 and 3,400 Hz.
A typical passband 10 for a speech communication system is shown in FIG. 1. In general, passband 10 is adequate for delivering speech signals that are both intelligible and are a reasonable facsimile of a person's speaking voice. Nonetheless, much speech information contained in higher frequencies outside the passband 10 is lost due to bandpass filtering. This can have a detrimental impact on both intelligibility and quality in environments where significant amounts of noise are present.
In many cases, the quality of band-limited signals can be improved by reintroducing the harmonic components of signals that have been lost because they lie outside of the system's passband. In some systems, such as that disclosed in a co-pending U.S. patent application Ser. No. 11/110,556, entitled “System for Improving Speech Quality and Intelligibility,” the entire disclosure of which is incorporated herein by reference, higher frequency components of speech signals are transposed or compressed into lower frequency ranges that are within the system's passband. In this case the compressed speech signals retain much of the information from the higher frequency ranges that are outside the passband and which would otherwise be lost if the signal were not compressed. This step alone significantly improves the quality and intelligibility of band-limited speech signals. Nonetheless, such frequency compressed signals experience further significant quality and intelligibility improvements if they are re-expanded after they have been transmitted over the narrowband communication channel and harmonics have been reintroduced at higher frequencies.
Presently, several techniques exist for extending the frequency range of harmonic signals for both speech and music. In many cases extending the harmonic signal content may be described as “excitation signal generation.” These techniques can be broadly grouped into two categories: frequency shifting methods; and nonlinear distortion methods.
Frequency shifting methods involve some form of spectral copying, transposition, or folding, in order to introduce a replica of lower frequency harmonics at higher frequencies. Many of these methods use a fixed copying scheme, which can result in the improper placement of the high-frequency harmonics. In many cases, the re-introduced high frequency harmonics will not be placed accurately at each multiple of the fundamental pitch frequency. Some spectral copying methods use a pitch estimate to insure the proper placement of transposed harmonics. However, performance of these methods can become severely degraded if the pitch estimate is inaccurate. This is often the case with signals having a low SNR.
The second category of harmonic extension methods involves creating harmonic distortion so that harmonics are introduced across the full frequency spectrum. These methods employ a time domain non-linear transformation such as a squared function x2(n), cubic function x3(n), or full-wave rectification |x(n)|, to introduce harmonic distortion. These methods are usually followed by spectral envelope estimation techniques, such as linear prediction, which are used to ensure that the final wideband excitation signal is spectrally flat.
The main advantage of non-linear transformation methods over spectral copying or folding methods is that harmonics are generated at multiples of the fundamental frequency without requiring the use of a pitch estimation algorithm. However, the main disadvantage of these techniques is that the new harmonics can contain aliasing artifacts in the higher frequencies. Also, because it is a time domain approach, it is difficult to control the bandwidth of the generated harmonics. New harmonics are generated across all frequencies instead of only the frequency range of interest.